Water-in-oil emulsions (oil-mud) are often used in circulating fluids required in the rotary drilling of formations containing hydrocarbons. The circulating fluids are referred to as drilling muds. The common specification of the term “mud” is defined according to its usual meaning in the oil and gas drilling industries, namely to describe a drilling fluid used to transport rock cuttings from a wellbore.
Such fluids or mud formulations are typically tailored to specific well bore conditions and are costly to formulate. The most expensive component of oil-mud systems is the hydrocarbon or synthetic base oil used as the fluid matrix for such systems. One objective in the process of drilling is, therefore, to conserve drilling muds, and where possible recycle and reuse the base oil.
Oil-mud circulating fluids are pumped down the drill pipe and out into the wellbore through holes in the drill bit and are recirculated back up the well in the annular space between the drill pipe and walls of the well bore, carrying with it drill cuttings and the like that are then removed before recirculation. The mud performs a number of functions, including removing drill cuttings, lubricating and keeping the bit cool, providing flotation to help support the weight of the drill pipe and casing, providing hydrostatic pressure to prevent caving in and undesirable flow of fluids and/or gases in or out of the well bore, including drilling fluids, brine, and the like.
The properties and composition of the drilling mud formulations are complex and variable, depending on the conditions involved and the results desired or required including the ability to reuse and recycle the mud formulations. In oil mud-drilling fluids, the oil, hydrocarbon or synthetic, is the continuous phase and the water is present in a dispersed phase. This is necessary to maintain the required rheology of the mud for drilling and completion, including a balance between gel strength and viscosity, i.e., the balance for example between pumpability of the mud and its hole cleaning capability. Further, it is necessary to maintain the oil phase as the external phase in order to keep the drilled solids oil-wet to prevent the solids from coming in contact with water and easily dispersing in the mud.
Mud solids include particles that are drilled from the formation, material from the inside surface of the hole and materials that are added to control the chemical and physical properties of the mud, such as weight material, including but not limited to barite and calcium carbonate and the like. Drilled solids' particles are created by the crushing and chipping action of rotary drill bits. Additional solids enter the well bore by sloughing from the sides of the open hole. From the time they enter the well until they reach the surface, drilled solids particles are continuously reduced in size by abrasion with other particles and by the grinding action of the drill pipe.
If mud solids are not properly controlled, the mud's density can increase above its desired weight and the mud can get so viscous that it creates difficulties in pumping the fluid. The increase in density can become critical if the density exceeds the pore pressure of the formation which can lead to loss of the fluid and increased costs related thereto. Since the earliest days in the oil industry, drillers have been trying to combat high solids content through the use of settling pits. However, some drilled solids are so finely ground that they tend to remain in suspension regardless how long they are allowed to settle. The fine solids in suspension result in increased mud viscosity and gel strength, which in turn results in larger particles also remaining in suspension. Thus, the approach of removing cuttings through settling alone is of limited practical value.
Of primary detriment to drilling fluids are ultra fine (5 μm to 30 μm) and colloidal (0.1 μm to 5 μm) size drill cutting particles. These small particles, if not removed, create havoc in a variety of ways. They slow the rate of penetration (ROP) compounding the number of rig days required and the cost. They disrupt the drilling fluid rheology, especially gel strengths, thereby upsetting the Equivalent Circulating Density (ECD), risking mud losses and worse, creating potential blow out situations. Ultra fine particles and colloidal particles cannot typically be removed by shale shakers, de-sanders, de-silters, or mud cleaners. Conventional centrifuges can typically only remove solids down to about 10-20 μm. Under conventional methodologies, ultra fine particles and colloidal particles create the need for excessive dilution to control the fluid density thus escalating mud cost and worse, contributing to environmental disposal problems by excessive mud build-up.
Based on studies done in the early 1970's, ultra-fine colloidal solids have the most detrimental reduction effect on ROP. Research has demonstrated that doubling colloidal content, though it may be only a relatively small part of overall solids volume, can reduce ROP by as much as 70-80%. As the solids surface area grows, water demand and chemical demand of the fluid grows, exponentially increasing demand for hydraulic horsepower, driving up plastic viscosity and creating sticky, spongy cakes on the wellbore walls.
Further, ultra-fine solids are not the only contaminants of invert-emulsion drilling muds. Additional aqueous fluids that can be introduced into these systems can lead to significant difficulty in the recovery of the base oil fluid. During the drilling process, it is also not uncommon for the drilling fluid to encounter a water bearing formation and thus, the ratio of aqueous fluid to non-aqueous fluid is less than optimal. In some cases, the formation of a difficult-to-break emulsion occurs and this is often referred to as the “slop”. The oil to water ratio in the slop may be 25/75 or 30/70 or similar such numbers. Using conventional methods of emulsion breaking it is possible to recover, for example, a 60/40 ratio of oil to water fluid. The recovered oil is then diluted with additional make up oil to achieve the desired ratio, being typically 80/20. One of the primary difficulties associated with this system is the use of emulsion breakers and surface tension breakers that are not environmentally friendly. There are many citations in the literature that deal with the separation of the excess aqueous fluids from invert-emulsion drilling fluids, such as U.S. Pat. No. 6,881,349 to Mueller and U.S. Pat. No. 6,977,048 to Mueller.
Drilling fluids made up of expensive polymers and oil base synthetics demand high performance decanters to control excessive mud cost. As the drilling parameters become more and more complex, involving high temperature additives, etc., the need to remove ultra fine and colloidal particles from the mud becomes paramount.
Drilling performance is typically optimized by the use of oil based or synthetic based mud. However, in an attempt to make these technologies more commonplace and acceptable from an economic standpoint, there is a definite need to be able to use the fluid systems as many times as possible. Further, there is a need to avoid major environmental issues created by the disposal of waste material generated from the use of these systems.
There are a number of different methodologies noted in the literature for cleaning of drill solids, but very little has been done to address the recovery and reuse of the hydrocarbon base oil from an economic standpoint, which may lead to reduced oil and gas exploration costs.